1. Field of the Invention
The present invention relates to a plasmon antenna that generates near-field light by receiving light, and to a heat-assisted magnetic recording head for writing data with the plasmon antenna by heat-assisted magnetic recording technique.
2. Description of the Related Art
As the recording density of a magnetic recording apparatus, such as a magnetic disk apparatus, becomes higher, further improvement has been required in the performance of a thin-film magnetic head and a magnetic recording medium. As the thin-film magnetic head, a composite-type thin-film magnetic head is widely used, which has a stacked structure of a magnetoresistive (MR) element for reading data and an electromagnetic transducer for writing data.
Whereas, the magnetic recording medium is generally a kind of discontinuous body of magnetic microparticles gathered together. Here, one record bit consists of a plurality of the magnetic microparticles. Therefore, in order to improve the recording density, it is necessary to decrease the size of the magnetic microparticles and reduce irregularity in the boundary of the record bit. However, the decrease in size of the magnetic microparticles raises a problem of degradation of thermal stability of magnetization due to decrease in volume.
As a measure against the thermal stability problem, it may be possible to increase the magnetic anisotropy energy KU of the magnetic microparticles. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. Whereas, write field intensity of the thin-film magnetic head is limited by the amount of saturation magnetic flux density of the soft-magnetic pole material of which the magnetic core of the head is formed. Therefore, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field (coercive force) of the medium exceeds the write field limit.
Recently, as a method for solving this problem of thermal stability, so-called a heat-assisted magnetic recording technique is proposed, in which writing is performed by reducing the anisotropic magnetic field with heat supplied to the magnetic recording medium formed of magnetic material with a large KU just before application of the write field. For the heat-assisted magnetic recording technique, a method has been intensively developed, in which a near-field light probe, so-called a plasmon antenna, is used for generating near-field light, and the near-field light is irradiated to the magnetic recording medium. For example, U.S. Pat. No. 6,768,556 B1 discloses a near-field light probe that includes a metal scatterer with a strobilus shape formed on a substrate and a dielectric material film formed around the metal scatterer. And U.S. Pat. No. 6,649,894 B2 discloses a near-field light probe made of a flat scatterer formed on the surface of a substrate. Further, US Patent Publication No. 2004/0085862 A1 discloses a near-field light probe that has a beak portion directed toward a magnetic recording medium, so-called a nano-beak type near-field light probe.
However, when the heat-assisted magnetic recording is intended to be realized by using such a plasmon antenna as described above, a difficult problem, which will be described hereinafter, may occur.
Generally when a plasmon antenna, which is a polygonal flat plate, is irradiated with laser light, near-field light is generated at the location of each of the corners. For example, it has been confirmed that when a flat plate with the shape of a triangle ABC is irradiated with a TM polarization light having an electric field component perpendicular to the bottom edge of the triangle, a near-field light having about a half the intensity of the near-field light generated at a vertex A is generated at each of vertexes B and C which is at both ends of the bottom edge. Specifically, the near-field light intensity ratio is such that vertex A:vertex B:vertex C=2:1:1.
However, it is ideal that the near-field light to be irradiated to a magnetic recording medium is generated at only one spot in a plasmon antenna. By using the near-field light that is generated at only one spot, only the desired record bit can be formed. On the contrary, for example, if a near-field light with a considerable intensity is generated at each of the three vertexes as described above, there arises the fear that two record bits other than the desired record bit are formed on the magnetic recording medium. In order to form only the desired record bit, the intensity ratio of the desired near-field light and each of the incidentally-generated near-field light is actually required to be at least 5:1. Thus, it has been difficult to realize the plasmon antenna in which only the desired near-field light among the generated near-field lights has a sufficient intensity.